Injection apparatus and injection control method for high-speed thin plate continuous casting machine

ABSTRACT

An injection apparatus for a high-speed type thin plate continuous casting machine comprises linear motors (3A, 3B) interposing long sides of a flat nozzle (3), a power source unit for applying predetermined currents having a predetermined frequency to the linear motors (3A, 3B), and linear motor power factor improving capacitors (21) connected between the linear motors (3A, 3B) and the power source unit. A material on the inner walls of the short sides of the flat nozzle (3) is a conductive material to improve an edge effect, additionally, the apparatus is constructed to use a heating operation of the linear motor in heating the nozzle or a molten metal in the nozzle by adequately controlling a frequency or a current of a power supplied to the linear motors.

DESCRIPTION

1. Technical Field

This invention relates to an injection apparatus and an injection ratecontrol method for injecting molten metal into a casting mold in ahigh-speed type thin plate continuous casting machine which cancontinuously cast into a thin strand at high speed.

In this type of apparatus, for example, thin steel plates having athickness of about 40 mm are directly produced from molten steel, sothat a process of manufacturing a steel plate can be rationalized.However, it is necessary to pull the strand out at high speed in orderto increase the productivity (ton/hour) because the thickness of thestrand is so thin.

The present invention relates to an injection apparatus and an injectioncontrol method for injecting molten metal into a casting mold in a thinplate continuous casting machine which can cast at high speed.

2. Background Art

Generally, in a continuous casting plant, it is important to keep amolten metal level constant within a casting mold, to stabilize thequality of a stand, especially to stabilize the trace in the surface ofthe strand, and to prevent the molten metal from overflowing in thecasting mold and damage the plant. Therefore, in a conventionalcontinuous casting plant, the molten metal level is controlled bydetecting the molten metal level within the casting mold using a moltenmetal level sensor, such as a sensor utilizing the principle ofelectromagnetic induction, and adjusting the injection rate by moving astopper filling a hole provided in bottom of a tundish up or down or byopening or closing a sliding nozzle.

In the aforementioned thin plate continuous casting plant, it isrequired to raise a pulling-out rate of the stand to five to ten timesas much as in a general continuous casting machine to achieve productionequal to the general continuous casting machine, because of the smallcross section, as mentioned before. In that case, as fluctuation in themolten metal level in the casting mold is frequent and violent, it isrequired to control the level using an apparatus having quick response.

Therefore, development of a molten metal level detecting means and ainjection rate control means which have far quicker response than amolten metal level sensor or injection rate control unit usually used,is required to realize a high-speed type thin plate continuous castingmachine to which the present invention relates.

Various molten metal level detecting means having quick response havebeen proposed (for example, molten metal level detection using alight-sensitive element described in Examined Patent Publication(Kokoku) No. 62-52663 On the other hand, an injection rate control meansto which the principle of electromagnetic force is applied, is promisingas an injection rate control means having quick response.

Three kinds of systems, i.e., a direct current static magnetic field(electromagnetic brake) system, a current flowing (forced direct currentplus direct current static magnetic field) system, and a linear motor(alternating current moving magnetic field) system are known at present.The present inventors grasped various characteristics of the threesystem through experiments, theoretical calculations, and literature,and compared and examined those characteristics. The present inventorsthen found that the linear motor is most suitable for the injectionapparatus for a thin plate continuous casting machine.

The reason for the selection is as follows. In the direct current staticmagnetic field system, the molten metal cannot be accelerated andheating characteristics are not sufficient. In the current flowingsystem, the apparatus is large and complicated, and interferes with theoperator's work. Furthermore, there is some anxiety regarding the safetyof the system.

Researching prior arts relating to linear motors, Japanese ExaminedUtility Model Publication No. 44-17619 was found as a publication whichdiscloses an application of a linear motor to a continuous castingmachine. The publication discloses a technique where a tundish isdivided into two vessels between which a linear motor is arranged tocontrol a molten metal level of the vessel situated above a nozzle. Inthis system, however, the response is not fast since the molten metal isinjected into a costing mold through the vessel situated above thenozzle after its injection rate is controlled by the linear motor.

It is assumed that the reason this configuration was adopted is that ifthe linear motor was used with the conventional nozzle having a circularcross section, effective control would not be carried out because theefficiency of the electromagnetic force is not sufficient.

Nevertheless, realization of an injection rate control means havingquick response is a dream which engineers can not abandon. JapaneseUnexamined Patent Publication (Kokai) No. 60-99458 discloses a linearmotor used with a conventional (circular) nozzle. In this prior art, anormally conducting coil and a superconducting coil are arranged besidea circular nozzle in an arrangement where fluxes of the coils do notinterfere with each other, to increase the electromagnetic force.However there are problems in the prior art that the length of thenozzle has to be long, and maintaining a very low temperature (below 4°K. in metal, below 100° K. in ceramics) to maintain the superconductivestate is difficult.

Therefore, the application of the linear motor to the thin platecontinuous casting machine seems to still remain at the stage of beingonly an idea. The basis of this inference is that the fact that successin utilization has not been reported yet and no information thatexploitation of this approach is progressing is known. In a word, thegoal in applying a linear motor to an injection apparatus for a thinplate continuous casting machine is the development of a practicallinear motor unit. The first problem to be examined is an improvement inthe efficiency of the electromagnetic force acting on the molten metal.

If the efficiency is improved, the size of the linear motor and itspower consumption can be reduced. As a result, the length of theinjection nozzle can be shortened, so that the production yield of thenozzle is improved.

Examining ways of improving the efficiency of the electromagnetic forcefrom theoretical calculations and repeated experiments, the presentinventors acquired the following knowledge:

A. The efficiency is raised when the width of the gap between the linearmotors arranged beside the injection nozzle is reduced.

B. The efficiency is raised when the distance between the inner walls ofthe nozzle along the direction of the width of the linear motors isenlarged to reduce the influence of the edge effect which is anelectromagnetic phenomenon.

As a result of the present inventors acquiring the above knowledge, itwas found to be most preferable that the injection nozzle used with thelinear motors should have a flat or rectangular cross section, thedistance between the pair of linear motors is reduced to reach thelength of the short sides of the flat nozzle, and the linear motorsshould be arranged so as to align the direction of the resulting edgeeffect with the direction of the long sides of the flat nozzle.

On the other hand, although not described for use with linear motors, aso-called flat nozzle having a flat or rectangular cross section isdisclosed in

Japanese Unexamined Patent Publication (Kokai) No. 60-12264.

However, several problems still remain in the case where a combinationof the aforementioned flat nozzle and linear motors is applied to thethin plate continuous casting machine. The first problem is powerconsumption. Since the flat nozzle is required to have a strength, theflat nozzle must have a sufficient thickness. Therefore, the distancebetween the linear motor and the molten steel in the nozzle, namely, thegap, is large so that reactive power is large due to a large leakagereactance.

The second problem is the edge effect. Distribution of theelectromagnetic force is not uniform along the direction of the longside, namely, the direction perpendicular to both direction of themagnetic field and the direction of injecting the molten metal. Theelectromagnetic force is maximum at the center part and extremelyreduced at the edge part. Therefore, the molten metal flow near the edgepart cannot be sufficiently controlled at present.

Additionally, the linear motor not only has the effect of theelectromagnetic force but also has an effect of heating It isanticipated to utilize this effect in the continuous casting plant.

The present inventors investigated the aforementioned first problem,that is, the problem of power consumption. As a result of theinvestigation it was found that reducing the reactive power improves thepower factor, and that it is most preferable to arrange a power factorimproving capacitor near the linear motor as a measure to achieve thatimprovement. Accordingly, when applying the linear motor to theinjection apparatus for a thin plate continuous casting machine, theflat nozzle and the power factor improving capacitor may be necessaryelements.

In order to control the force of the linear motor acting on the moltenmetal, current or voltage is mainly controlled, keeping the frequencyconstant so as to maintain the effect of the power factor improvingcapacitor. However, if the frequency has to be altered, it is preferableto alter the capacitance of the power factor improving capacitordepending on the frequency.

Regarding the second problem of the edge effect, the present inventorssolved this problem by devising a flat nozzle as described laterHowever, this device is not necessary, but only preferable inconstruction.

Furthermore, the present inventors found that the following two methodsare adequate for simultaneously generating the acting force and theheating effect of the linear motor.

The first method is deciding the frequency and the current (or voltage)of the supplied power to the linear motor according to a specificcondition, in the case where the acting force and the heating by thelinear motor are applied to the molten metal. In this case, thecapacitance of the power factor improving capacitor is varied byswitching.

The second method is superimposing a plurality of powers havingdifferent frequencies as the power applied to the linear motor. Thismethod is described later in detail.

DISCLOSURE OF THE INVENTION

Accordingly, it is a primary object of the present invention to providea practical injection apparatus for a high-speed type thin platecontinuous casting machine, comprising a linear motor arranged close toa flat nozzle, which can solve the aforementioned problems to control aninjection rate at high efficiency and with quick response and whichconsumes little electric power.

Additionally, it is a secondary object of the present invention toprovide an injection apparatus for a high-speed type thin platecontinuous casting machine, which effectively utilizes the heatingeffect of the linear motor.

It is another object o the present invention to provide a control methodof an injection rate of a molten metal in the aforementioned injectionapparatus.

The primary object is carried out by an injection apparatus for ahigh-speed type thin plate continuous casting machine wherein a moltenmetal is injected into a casting mold from a tundish through a flatnozzle having long sides in a Y-direction longer than short sides in anX-direction and elongated along a Z-direction, characterized in that theinjection apparatus comprises:

linear motors, positioned between the long sides of the flat nozzle forgenerating an electromagnetic feed force in a z direction along the longsides;

a power source unit for applying predetermined voltages or currentshaving a predetermined frequency to the linear motors to cause thelinear motors to generate an electromagnetic feed force; and

linear motor power factor improving capacitors connected to an electricline between the power source unit and the linear motors.

It is preferable that the apparatus further comprises power controlmeans inserted between the power source unit and the linear motors, forcontrolling at least one of the voltages and currents supplied to thelinear motors to control a Z direction acceleration/deceleration forceacting on the molten metal in the flat nozzle.

It is also preferable that the inner walls of the flat nozzle in theshort side essentially consist of a conductive material which is durableagainst the molten metal.

The secondary object is carried out by an injection apparatus furthercomprising:

a temperature detecting means for detecting a temperature of the moltensteel,

a calculation unit for calculating a heat quantity Q supplied to themolten steel by the linear motors and a force P from the linear motorsacting or the molten steel from the signal of the temperature detectingmeans, and further calculating a frequency f and a current i using aformula

    f=K.sub.1 (Q/P)

and ##EQU1## wherein K₁ and K₂ are constants, and a power convertingunit for converting commercial power to a power having a frequency f anda current i according to the output of the calculation unit andsupplying the power to the linear motors.

Another object of the present invention is carried out by a methodwherein at least one of a voltage and current supplied to the linearmotors is adjusted to control the injection rate from the flat nozzle tothe casting mode in the aforementioned apparatus.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a diagram showing an outer appearance of a whole vertical thinplate continuous casting machine according to the present invention;

FIG. 2 is a diagram showing a first embodiment of an injection apparatusaccording to the present invention;

FIG. 3 is an enlarged view of a flat nozzle and linear motors;

FIG. 4 is a cross-sectional view of an apparatus formed by modulatingthe apparatus shown in FIG. 2;

FIG. 5 is a longitudinal sectional view of an apparatus formed byanother modulation of the apparatus shown in FIG. 2;

FIG. 6a and 6b is a flow chart showing a process in microcomputer 30 inthe apparatus shown in FIG. 2;

FIG. 7A is a diagram representing a second embodiment of the injectionapparatus according to the present invention;

FIG. 7B is a detailed diagram of the embodiment illustrated in FIG. 7Ashowing a sliding nozzle.

FIG. 8 is a diagram representing a third embodiment of the injectionapparatus according to the present invention;

FIG. 9 is a diagram representing a frequency distribution oflow-frequency power L and high-frequency power H in the apparatus shownin FIG. 8;

FIG. 10 is a diagram representing a relation between a frequency f of apower and a permeation depth δ;

FIG. 11 is a diagram representing a fourth embodiment of the injectionapparatus according to the present invention;

FIG. 12 is a diagram representing a fifth embodiment of the injectionapparatus according to the present invention;

FIG. 13 is a diagram showing a cross section of a flat nozzle in theinjection apparatus according to the present invention;

FIG. 14a is a diagram for explaining edge effect in the prior art;

FIG. 14b is a diagram representing an improvement of the edge effect inthe apparatus according to the present invention;

FIG. 15 is a diagram representing a sixth embodiment of the injectionapparatus according to the present invention;

FIG. 16 is a block diagram representing control in the apparatus shownin FIG. 15;

FIG. 17 is a diagram representing a seventh embodiment of the injectionapparatus according to the present invention;

FIG. 18 is a diagram representing response in control by a linear motorand in control by a sliding nozzle;

FIG. 19 is a diagram representing response at a various casting rates;and

FIG. 20 is a diagram representing an experimental result of flow ratecontrol in the injection apparatus according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a schematic diagram representing an entire thin platecontinuous casting machine, to which the present invention is applied,and FIG. 2 is a diagram representing the construction of an injectionapparatus according to the present invention.

A molten metal 2 in a tundish 1 is injected into a casting mold througha flat nozzle 3 having a rectangular cross section having a small widthin an X direction and a large width in a Y direction perpendicular tothe X direction.

In this embodiment, the casting mold is a dual belt type casting moldconstituted by two casting belts 4 (only the forward casting belt isshown in FIG. 1) opposite to each other to interpose the nozzle 3, andtwo movable short sides 13 opposite to each other to interpose thenozzle. Each belt 4 has a width larger than the width (Y direction) ofthe long side of the flat nozzle 3. The short side 13 has a width largerthan the width (X direction) of the short side of the flat nozzle 3.

The short side 13 is described in detail in Japanese Patent ApplicationNos. 62-328080 and 62-328082.

In all examples of the injection apparatus according to the presentinvention, the longitudinal direction (Z direction) of the flat nozzle 3is designed to be vertical. Thus, the flow rate can be larger thandesigned to be inclined, so that it become easy to smoothly control by alinear motor by filling the nozzle with the molten metal. Additionally,a stopper or a sliding nozzle (not shown) to adjust the injection rateof the molten metal is provided.

The casting belts 4 are suspended and supported by driving rollers 5,5'. The driving rollers 5, 5' are driven by a DC motor 7 through areduction gear mechanism 6 at a predetermined speed. A designation speedgenerator (pulse generator or tachogenerator) 8 is connected to themotor 7. For example, in the case of the pulse generator, it generates apulsed voltage having a frequency proportional to the speed of the motor7. This pulsed voltage is converted by a pulse processing circuit 11into a pulse signal having a frequency proportional to the frequencygenerated by the pulse generator and a predetermined pulse amplitude andwidth. An F/V converter 12 generates a voltage (speed voltage) having alevel proportional to the above frequency. The motor driver 9 controlsan armature current on the basis of a target speed (voltage) suppliedfrom the motor controller 10, the feedback speed (voltage) applied fromthe F/V converter 12, and the armature current (torque) of the motor 7so that the actual speed of the motor 7 reaches the target speed. Themotor 7 is then rotated at the target speed designated by the motorcontroller 10. That is, the belts 4 are driven at the target speed.

A pair of linear motors 3A and 3B are arranged to interpose the longsides (Y direction) of the flat nozzle 3. The relationship between thelinear motors and the flat nozzle 3 is shown in FIG. 3.

The linear motors 3A and 3B have a shape wherein a stator of a 3-phasestar-connected induction motor is developed on a plane. The respectivephase coils are stored in slots between magnetic poles opposite to therotor (molten steel in the nozzle 3). When 3-phase AC components havinga predetermined phase relationship are applied to the phase coils, anupward electromagnetic feed force (deceleration force) in the Zdirection is generated in the molten steel. When the AC voltagecomponents applied to the two phase electric coils are reversed, adownward electromagnetic force (acceleration force) in the Z directionis generated in the molten steel in the nozzle 3.

FIG. 4 is a diagram representing in detail a cross section bycutting-off an apparatus formed by modulating the apparatus shown inFIG. 2, at the center of the linear motor 3A and 3B with a planeperpendicular to the Z-direction. FIG. 5 is a cross section of anapparatus formed by modulating the apparatus shown in FIG. 2, similarlyto FIG. 2. The state of windings belonging to the respective phase isshown in detail in FIG. 5. The same reference numerals as used in FIG. 1to FIG. 3 are used in FIG. 4 and FIG. 5 for constituents which aresimilar to those in FIG. 1 to FIG. 3.

Returning to FIG. 1 to FIG. 3, the phase coils of the linear motors 3Aand 3B are connected to the respective phase output lines of a 3-phaseAC power source circuit 24 through a thyristor inverter 23 forcontrolling bidirectional conduction and a phase order switching circuit22 in units of lines. The thyristor inverter 23 is turned on in responseto an ON trigger pulse from a thyristor driver 25 at positive halfcycles of the AC voltage to apply the respective AC phase voltages tothe linear motors 3A and 3B, and is turned off at zero-crossing pointsof the AC voltage.

Power factor improving capacitors 21 are connected to connecting linesbetween the respective phase coils of the linear motors 3A and 3B andthe respective phase lines of the 3-phase AC voltage components toreduce the aforementioned reactive power. In this embodiment, since thefrequency of the 3-phase AC voltage preferably falls within the range of100 to 500 Hz to minimize eddy current loss in the molten steel in thenozzle 3, this frequency is set to 120 Hz. That is, the 3-phase AC powersource circuit 24 outputs 120-Hz AC voltage components having 120° phasedifferences to the respective 3-phase output lines. The total power ofthe linear motors 3A and 3B is 2,800 kVA at 120 Hz. The capacitors 21have a power of 2,800 kVA accordingly. In a conventional arrangement,the required power of the inverter 23 is 2,800 kVA. However, accordingto the present invention, connections of the capacitors 21 greatlyreduce the power of the inverter 23 to 1,200 kVA, thereby additionallyreducing the power source equipment cost.

Thus a linear motor having the power factor improving capacitors hassuch a high efficiency that the capacity of the power source can bereduced, however there is a factor which must be considered in using thecapacitors. This is the fact that as the efficiency is altered when thefrequency of the voltage supplied to the linear motor is altered, thefrequency must fall within a narrow range.

Accordingly, there are two way to control the output power of the linearmotor. One is controlling current and/or voltage while the frequency isfixed, and the other is altering the capacitance of the power factorimproving capacitors through change-over switches to alter thefrequency. The inventors employed the former approach based on theirdiscoveries and consider that the latter approach should be used only inthe special case where both current and frequency must be altered at thesame time, as mentioned later.

A video camera 28 is arranged below the linear motor 3A to detect themolten steel level (the distance from the video camera 28 to the moltensteel surface) L_(d). The video camera 28 picks up an image of a portionof the movable short side 13 which is in contact with the molten steelsurface. The video signal from the video camera 28 is supplied to thesignal processing circuit 29. The signal processing circuit 29 extractsthe boundary (i.e., the high-temperature color portion on the imageobtained by picking up the image of the inner surface of the movableshort side) between the molten steel surface and the movable short side.The extracted boundary is determined whether to be located at an upperor lower position on the screen, and the distance L_(d) is calculated.Data representing the distance L_(d) is supplied to the microcomputer(referred to as the MPU hereinafter) 30. The MPU 30 receives thestart/end signal, the data representing the target injection rate (speedin the nozzle 3) V_(O), and the target level L_(O) (target value of thedistance from the video camera 28 to the molten steel) from a hostcomputer or operation panel (not shown). The pulse obtained byfrequency-dividing the speed pulse (i.e., the output pulse from thepulse processing circuit 11) is supplied from the frequency divider 31to the MPU 30.

The MPU 30 calculates a difference dL between the target level L_(O) andthe detection level L_(d) supplied from the signal processing circuit 29and then calculates the speed V_(i) of the molten steel injected intothe casting mold so as to nullify the difference dL. The MPU 30 alsocalculates linear motor energization current values for obtaining thespeed V_(i), and converts the calculated result into an ON angle (i.e.,a phase angle to make an ON state) of the thyristor converter 23. TheMPU 30 then supplies voltage data V_(f) representing the ON angle to thethyristor driver 25. The thyristor driver 25 generates a voltagegradually increased in proportion to an increase in AC voltage phase byusing zero-crossing points as reference points. This voltage is comparedwith the analog voltage V_(f). When the voltage from the thyristordriver 25 reaches the analog voltage V_(f), the thyristor driver 25generates a trigger pulse. The trigger pulse is supplied to the gate ofthe thyristor of the converter 23. Upon reception of this trigger pulse,the thyristor is turned on and then turned off at the next zero-crossingpoint.

FIGS. 6a and 6b show control operations of the MPU 30. First, theoperations will be described with reference to FIG. 6a. When a powerswitch is turned on (step 1s: the term "step" is omitted within theparentheses hereinafter), the MPU 30 sets the input/output ports in thestandby signal level and clears the internal registers, a counter, atimer, and the like. The MPU 30 sends a "ready" signal to the hostcomputer or operation panel. The CPU 30 then waits until control data(data for determining control parameters such as operation constants andtiming constants) and a start signal. When the control data are sent tothe MPU 30, it fetches these data and writes them in predeterminedregisters (internal RAM) (2S and 3S).

When the start signal reaches the MPU 30, the MPU 30 enables aninterrupt INT (4S), and causes a timer TO (i.e., a program timer forcounting the time interval TO) to start. The MPU 30 waits for a time-outof the timer TO (5S and 6S).

When the interrupt INT is enabled, the MPU 30 executes interruptprocessing shown in FIG. 6b every time the frequency divider 31generates one pulse, and this operation will be described below. Whenone pulse is generated by the frequency divider 31, the timer T_(O) isstarted (restarted) (10S), the MPU 30 reads the molten steel detectionlevel Ld and the molten steel target level L_(O) (11S and 12S). The MPU30 then calculates the difference dL, and the calculated value is storedin a register A_(cd) (13S and 14S). The difference dL is multiplied by aproportional constant K_(p), and the product is stored in a registerA_(c3) (15S). Data in accumulation registers R₁ to R_(n) are shifted toeliminate the oldest data (R_(n)) so that the data of the registerR_(n-1) is stored in the register R_(n), and the data of the registerR_(n-2) is stored in the register R_(n-1) (16S to 18S). A productobtained by multiplying the difference dL by an integral constant K_(i)is stored in the empty register R₁ (19S). A summation (i.e., an integralamount of the correction value) of the data of the registers R₁ to R_(n)is obtained and written in a register A_(c4) (20S). The molten steelspeed V_(i) in the nozzle 3 as a PI control output value is calculated(21S). The ratio V_(r) of the predetermined speed V_(i) to the targetspeed (proportional to the casting target rate) V_(O) in the nozzle 3 iscalculated, and the calculated result is stored in a register A_(c5)(22S). Linear motor current data I_(i) corresponding to the ratio V_(r)is read out from the data table which is prestored in the internalmemory, and the readout data is stored in a register A_(c6) (23S). TheON phase angle data V_(f) for producing the current I_(i) is read outfrom the data table prestored in the internal memory, and the readoutdata is stored in a register A_(c7) (24S). The MPU determines whetherthe data (correction value with respect to the target speed V_(O))stored in the register A_(c4) is positive or negative (25S), i.e.,whether the linear motors are to be accelerated or decelerated. If thedata is determined to be positive (acceleration), an H output issupplied to the relay driver 27 (27S). The relay contact of the phaseorder switching circuit 22 is driven downward, and the linear motors 3Aand 3B are connected to the inverter 23 so as to achieve acceleration(i.e., downward driving in the Z direction). If the data is determinedto be negative (deceleration), an L output is sent to the relay driver27 (26S). The relay contact of the phase order switching circuit 22 islocated at the position shown in FIG. 2. In this state, the linearmotors 3A and 3B are connected to the inverter 23 to achievedeceleration (i.e., upward driving in the Z direction). The MPU 30updates the data V_(f) of the register A_(c7), and the updated data issupplied to the thyristor driver 25 (28S). As described above, thedriving direction and force of the linear motors 3A and 3B are correctedin correspondence with the detection value L_(d).

The above interrupt processing is performed every time the frequencydivider 31 generates one pulse. An integral value of the differencesobtained in previous n interrupt operations is stored in the registerA_(c4).

The time interval T_(O) of the timer T_(O) is slightly longer than aperiod T_(m) of a pulse generated by the frequency divider 31 when thecontinuous casting machine shown in FIG. 1 is set at a designed minimumspeed. Therefore, when the DC motor 7, the tachogenerator 8, the pulseprocessing circuit 11, and the frequency divider 31 are normallyoperated, a pulse is generated by the frequency divider 31 before thetime-out of the timer T_(O). The time-out of the timer T_(O) does notoccur. Therefore, the interrupt processing shown in FIG. 6b isrepeatedly performed in a normal state.

When no pulse is generated by the frequency divider 31 during the timeinterval T_(O) due to some abnormality, interrupt processing (FIG. 6b)is not executed, and the time-out of the timer T_(O) occurs. The MPU 30advances from step 6 to step 30 in FIG. 6a and sends an alarm signal tothe host computer or operation panel (30S). The timer T_(O) is started(restarted) (31S), and the MPU 30 performs an input read operation (S),a PI control output value calculation (S), a phase angle calculation(S), a driving direction calculation (S), and an output operation (S).The MPU 30 terminates a series of operation. The contents of theoperations (AS to ES) are the same as those of steps 11 to 28 in FIG.6b.

The PI control sampling period is determined by a pulse generated by thefrequency divider 31 so as to inverse-proportionally shorten thesampling period when the casting rate is high.

When an end signal is received from the host computer or operation panelto the apparatus (7S), the MPU 30 is advanced to step A, carries out theaforementioned steps, and is terminated, i.e., set in a standby state(the linear motors are stopped).

FIG. 7 is a Y direction sectional view of an injection nozzlerepresenting a second embodiment of the apparatus according to thepresent invention.

Reference numeral 13' denotes short-side members of a casting mold.Metal belts 4 are spaced apart from each other by, e.g., a thin steelplate with a thickness of about 40 mm between the upper and lowersurfaces of the drawing sheet of FIG. 7, and are driven in parallel toeach other at high speed in a direction indicated by an arrow 50.

A point P in FIG. 7 indicates a molten steel surface position on acosting mold wall surface (a position corresponding to theaforementioned L_(O)). The molten steel surface position serves as atarget in operation. Points Q and R indicate allowable upper and lowerlimits of the molten steel surface position in operation, respectively.

A molten steel position detection end according to the present inventionis constituted by an industrial television camera 28. The industrialtelevision camera 28 is installed to photograph images within the rangeof the positions Q to R. Only one industrial television camera 28 isinstalled in FIG. 7. However, a plurality of television cameras may beinstalled. The inner wall of the short-side of the casting mold whichopposes the television camera is used as an object to be photographed inFIG. 7. However, a conventional optical means may be used, and otherinner walls may serve as the objects. Since a portion near the moltensteel surface is exposed to high temperatures and there is a lot of dustat the portion near the molten steel surface, a molten steel surfacedetection end located near the molten steel surface may often be damagedor its detection precision may often be degraded. The television cameracan precisely detect the molten steel position even if it is installedaway from the molten steel surface. With this lay out, the televisioncamera is rarely damaged. In continuous casting for a thin steel plate,the gap between the long sides of the casting mold is very narrow, aspreviously mentioned. The industrial television camera is suitable fordetection of the molten steel surface position within this gap. Lightemission from the molten steel can be detected by other photosensitiveelements (CCD elements, etc.). However, the same visible image as theobject can be obtained by the industrial television camera. Therefore,the operations for adjusting the direction of the detection end so as tobe aligned with the object in prealignment can be facilitated.

Reference numeral 44 in FIG. 7 denotes a control unit. The televisioncamera is aligned so that the half of the image of the molten steelsurface at, e.g., the point P is bright on the industrial televisioncamera, the entire image at the point Q is bright, and the entire imageat the point R is dark. The signal processing unit 29 converts theseimages into signals. The signals are supplied to the control unit 44 andthe output signals of the control unit 44 are supplied to the linearmotor 3A and 3B and a stopper 15 (in detail stopper control unit; notshown).

An injection flow supplied from the nozzle 14 is free from disturbancebecause the nozzle 14 extending near or below the molten metal surfaceis used.

The apparatus of the present invention further comprises a stopper 15capable of closing the molten steel injection nozzle in response to thesignal from the control unit 44. As previously described, a large numberof traveling and pivotal components are used in the continuous castingmachine for a thin steel plate. For example, when the metal belts 4stops traveling due to a failure, a unit is required to quickly andaccurately stop molten steel injection so as to prevent the molten steelfrom overflowing from the upper portion of the casting mold. Althoughthe linear motor 3A and 3B is suitable for controlling the injectionrate of the molten steel, it is not suitable for perfectly stopping theinjection flow since a high static pressure of the molten metal in thetundish 1 acts on the nozzle 14, and also due to existing edge effect.For example, when the metal belts 4 stops, the molten steel surfacebecomes higher than the position Q. According to the present invention,when the molten steel surface position exceeds a dangerous range, thestopper 15 is operated in response to the signal from the control unit44 to stop the injection flow. When a casting accident caused by theoverflow of the molten steel from the upper portion of the casting moldoccurs, its repair is cumbersome. According to the present invention,this accident can be prevented by the stopper unit 15.

The stopper 15 may have a similar construction to that used in aconventional continuous casting plant to control an injection pate. Asliding nozzle used for the same purpose as the stopper in aconventional continuous casting plant can be used for the aforementionedpurpose. The sliding nozzle is not shown in the figures because it iswell known to those skilled in the art.

The stopper 15 or the sliding nozzle can be used for an emergency stopwhen the molten steel level exceeds an upper limit as mentioned above.In addition, control with the linear motors and control with either thestopper 15 or sliding nozzle can be used together to realize a systemwhere both controls compensate each other to realize only the merits ofboth controls. Namely, the control with the linear motors has anexcellent quality of quick response, but it cannot stop the injectioncompletely though a remarkable improvement is obtained according to thepresent invention. On the other hand, the stopper or the sliding nozzlehas a slow response, but has a wide control range including a completelystopped state.

Accordingly, if control with the linear motors is carried out when thedifference between the target molten metal level and the detected actualmolten metal level is smaller than a predetermined level, and controlwith the stopper or the sliding nozzle is carried when the differencebecomes larger than the predetermined level, then a control which hasquick response and a wide control range including a completely stoppedstate can be realized.

The predetermined value may be determined within the range where theinjection nozzle can endure an elevated force of the linear motors, asshown in FIG. 20. Though the greater predetermined value is suitable forcontrolling the molten metal level, it causes a higher degree of dangerof damage of the injection nozzle. Therefore, the value must bedetermined considering a balance of both factors.

In the case where the predetermined value is high, control of the moltenmetal level is usually carried out by operating the linear motors, andthe stopper or the sliding nozzle only serves to completely stop theinjection. Though the linear motors can stop the injection, the stoppedstate is not stable. Therefore, a stopped state over a long timeinterval should be performed with the stopper or the sliding nozzle. Ifthe predetermined value is very small, function of the linear motorsbecomes ineffective. Accordingly, it is preferable that employment ofthe linear motors be decided considering molten metal level fluctuationcharacteristics and the characteristics of the linear motors shown inFIG. 20.

FIG. 8 is a longitudinal sectional view showing a structure of a tundishand a portion near a casting mold in the continuous casting machine toexplain a third embodiment of the present invention. FIG. 8 shows astate during casting.

Referring to FIG. 8, an injection nozzle 3 extends from the bottomportion of a tundish 1 to the interior of a casting mold 26. Thecross-sectional shape of the injection nozzle 3 and the casting mold 26is rectangular. The injection nozzle 3 is made of alumina graphite. Apair of linear motors 3A and 3B are arranged to face both wide surfacesof the injection nozzle 3. Each linear motor 3A, 3B has a width largeenough to cover the opening of the injection nozzle 3 in the long-sidedirection of the casting mold 26.

A power supply unit 31 for supplying power to the linear motors 3A and3B comprises a low-frequency inverter 32, a high-frequency inverter 33,and power sources 34 and 35. The low and high-frequency inverters 32 and33 are connected to the linear motors 3A and 3B through a switch 16. Thelow-frequency inverter 32 and the switch 16 are controlled by a controlunit 36.

A permeation depth δ of an electromagnetic field in the conductor isexpressed by the following known equation (1) ##EQU2## where f is thefrequency of the power supplied to the linear motor, σ is conductivity,and u is permeability.

When the powers of appropriate frequencies f (frequencies of the high-and low-frequency ranges) are supplied to the linear motor in accordancewith the conductivities σ and the permeabilities μ of the molten metaland the injection nozzle, the electromagnetic field can be applied toonly the injection nozzle or both the molten metal and the injectionnozzle. Therefore, control of the injection rate and heating of theinjection nozzle can be performed by only the linear motors Since theconductivity of the molten metal is larger than that of the nozzle, thelinear motors serve as flow control units for applying a thrust to themolten metal upon reception of the low-frequency power. The windings ofthe linear motors serve as induction coils for heating the injectionnozzle upon reception of a high-frequency power.

As shown in FIG. 9, the low-frequency inverter 32 outputs alow-frequency power L, and the high-frequency inverter 33 outputs ahigh-frequency power H. The frequency of the low-frequency power isselected from the range of 30 to 3,000 Hz, and the frequency of thehigh-frequency power is selected from the range of 3 to 450 kHz. Morespecifically, when the relationship between the frequency f and thepermeation depth σ of the electromagnetic force is obtained on the basisof the conductivities σ and permeabilities u of the molten steel andalumina graphite in accordance with equation (1), molten steel M isrepresented by a line MM in FIG. 10, and alumina graphite is representedby a line N. When the actual thickness of the cast piece and the actualthickness of the injection nozzle 3 are taken into consideration, thepermeation depths δ of the electromagnetic fields for these thicknesspreferably fall within the range of about 10 to 100 mm. FIG. 10 showsthat the frequency ranges corresponding to these permeation depths δ are30 to 3,000 Hz for the low-frequency range and 3 to 450 kHz for thehigh-frequency range.

The technical specifications of the continuous casting machine havingthe above arrangement are as follows.

Casting mold (slab) sectional area: 600 mm (long side)×50 mm (shortside)

Injection nozzle outer dimensions: 300 mm (width)×30 mm (thickness)

No. of nozzles: 1

Injection nozzle outer dimensions: 300 mm (width)×30 mm (thickness)

No of nozzles: 1

Injection nozzle dipping depth: 50 mm

Casting rate: 10 m/min

The technical specifications of the linear motors are as follows.

Outer dimensions: 670 mm (height)×300 mm (width)×230 mm (thickness)

Winding groove dimensions: 80 mm (depth)×10 mm (width)×20 mm (pitch)

Rated low-frequency power: 400 kW at 120 Hz

Rated high-frequency power: 200 kW at 120 Hz

Control of the injection rate and heating of the injection nozzle areperformed in the continuous casting machine as follows.

Prior to casting, the switch 16 is switched to the high-frequencyinverter 33 to supply a high-frequency current to the windings of thelinear motors 3A and 3B, thereby performing induction heating of theinjection nozzle 3. At this time, since the injection nozzle is empty,only the injection nozzle 3 is heated. When the injection nozzle 3 isheated to a predetermined temperature, the control unit 36 switches theswitch 16 to the low-frequency side in response to a temperature signalfrom a temperature sensor 37. The molten metal M is supplied from thetundish 1 to the casting mold 26 through the injection nozzle 3.

The molten steel injection rate is changed in accordance with a moltensteel head in the tundish 1. When the cast piece S is flat, casting mustbe performed at a high casting rate and hence a high molten steelinjection rate. For this reason, the molten steel heat in the tundish 1and the molten steel injection rate are abruptly changed during progressof casting, and the molten steel surface level m is changed. However,the molten steel level m must fall within a predetermined range so as tostart cooling of the molten steel M from an optimal position in thecasting mold 26 and to prevent the molten steel M from overflowing fromthe casting mold 26. A molten steel surface level detector 14 arrangedabove the casting mold 26 detects the molten steel surface level m, anda signal therefrom is input to the control unit 36. The control unit 36instructs an output voltage applied to the low-frequency inverter 32 onthe basis of the level signal. As a result the output voltages appliedto the linear motors 3A and 3B are controlled, and hence the moltensteel level m can be maintained within the predetermined range. Switch16 is again switched to the high-frequency side when one injection cycleof the molten metal is finished. The injection nozzle and steel adheringto the inner wall of the injection nozzle are heated until the nextinjection cycle of the molten metal is started. In this way, continuouscasting is smoothly started without solidification and adhesion to theinner wall of the injection nozzle when the next cycle injection of themolten metal is started. If the heating effect of the linear motors isnot utilized, another means must replace it. However, another means isnot known at present.

FIG. 11 shows a fourth embodiment of the present invention.

In the above embodiment, the two inverters, i.e., the low-frequencyinverter 32 and the high-frequency inverter 33 are used to control theinjection rate and heat the injection nozzle. In the fourth embodiment,the above operations are performed by one inverter 38.

A power supply unit 39 comprises the inverter 38, a power source 40, anda control unit 41. In order to cause one inverter 38 to generate powerhaving a plurality of frequency components, a pulse-width modulationtype inverter is used to output rectangular wave voltages. An outputreference signal and a PWM-modulated signal input to the inverter 38 arecontrolled by the control unit 41, thereby controlling the outputvoltages and their frequencies. In this embodiment, control of theinjection rate and heating of the injection nozzle 3 are simultaneouslyperformed.

Accordingly, it is preferable that the linear motor be used forsimultaneous control of injection rate and heating of the injectionnozzle during injection of the molten steel, and be used for control ofonly heating of the injection nozzle before the injection and betweenthe injection. Heating of the nozzle is carried out in order to preventsolidification and adhesion of the molten steel or the like to the innerwall of the nozzle, gradually growing, and finally narrowing theeffective cross-sectional area of the nozzle. This is especiallyeffective in continuous casting.

FIG. 12 is a schematic side view of a casting mold and its periphery ina continuous casting machine showing a fifth embodiment of the presentinvention.

As shown in FIG. 12, a flat nozzle 3 extends from the bottom portion ofa tundish (not shown) to a molten metal M in a casting mold 26.

One of problems with employing linear motors in the injection unit of acontinuous casting machine is that the length of the injection nozzlemust be long. But because the injection nozzle is long, the productionyield become lower and the nozzle becomes liable to be damaged. Thelatter problem is serious because the force of the linear motors isadded to the pressure of the molten steel, and because if the injectionnozzle is damaged the linear motors are also damaged. Therefore,shortening of the injection nozzle as well as miniaturization of thelinear motors by improvement of the efficiency and improvement ofstrength of the injection nozzle, is a main design point.

The casting mold 26 comprises a pair of endless casting belts 4 woundbetween upstream rollers 5 and downstream rollers (not shown) and a pairof movable short sides 13 arranged at the left and right sides in awidthwise direction so as to oppose each other. The flat casting mold 26is formed so that the side surfaces of the movable short sides 13 are incontact with the belt surfaces.

A pair of linear motors 3A and 3B are arranged to face both widesurfaces of the flat nozzle 3. An iron core 17 of each linear motor 3A,3B, has a flat plate-like shape and an adequate width to cover anopening of the flat nozzle 3 with respect to the long-side direction ofthe casting mold. The iron core 17 has a plurality of grooveshorizontally extending to face the corresponding wide surface of theflat nozzle 3. Windings 18 are respectively arranged in the grooves togenerate a vertical traveling magnetic field when a current is appliedto the linear motor. The lower end of the iron core 17 is notched toextend along the circumferential surface of the corresponding upperroller 5 and is inserted between the flat nozzle 3 and the correspondingupstream roller 5. The windings 18 are arranged in even the lower endportion. A power source is connected to the windings 18 through aninverter (not shown), and an output from the inverter is controlled by acontrol unit (not shown).

The technical specifications of the dual belt type continuous castingmachine having the linear motors with the above arrangement are asfollows:

Casting mold (slab) selectional area: 600 mm (long side)×50 mm (shortside)

Nozzle outer dimensions: 300 mm (width)×40 mm (thickness)

No. of nozzle: 1

Nozzle dipping depth: 50 mm

Casting rate: 10 m/min

The technical specifications of the linear motor are as follows:

Outer dimensions: 670 mm (height)×300 mm (width)×230 mm (thickness)

Winding groove dimensions: 80 mm (depth)×10 mm (width)×20 mm (pitch)

Lower end portion insertion length L: 200 mm

Rated power: 2,800 kVA at 120 Hz

Pole pitch: 300 mm

Number of poles: 2

When the above linear motor was employed, the length of the flat nozzlecould be shorted by 200 mm as compared with the conventional flatnozzle. The effect is remarkable. As a result of casting by the abovecasting machine, the molten metal surface level could be maintainedalmost constant.

Next, distribution of an electromagnetic force particularly along thelongitudinal direction of the flat nozzle (Y-direction), and the edgeeffect problem are described.

The present inventors repeatedly made extensive studies and experimentsexcept for molten steel surface level control in which linear motors 3Aand 3B were arranged opposite to side surfaces of a flat nozzle 3, asshown in FIG. 13 (cross-sectional view). The present inventors confirmedthat phased silica and alumina graphite could not set the injection flowrate to zero due to a large edge effect in a refractory injectionnozzle. The present inventors tried to analyze this mechanism.

The linear motors 3A and 3B are arranged to oppose both sides surfacesof the injection nozzle 3. As shown in FIG. 13, a magnetic field B_(O)traveling as a function of time in a direction x of a molten iron flowis applied to a direction y perpendicular to the direction x of themolten iron flow. An electromagnetic force (the left-hand rule) by avector product between the applied traveling magnetic field and aninduction current depending on a traveling speed of the magnetic fieldB_(O) and a molten iron flow speed V is applied as an acceleration ordeceleration force in the direction x of the molten iron flow. When theelectromagnetic force is controlled, the flow rate of the molten iron ischanged. In order to change the electromagnetic force, the magnitude ofthe traveling magnetic field and its traveling speed are changed.Therefore, the magnitude of the traveling magnetic field of the linearmotor and the traveling speed of the magnetic field can be controlled byelectrical changes at high speed, thereby obtaining excellent responsecharacteristics.

When the linear motors 3A and 3B are arranged, as shown in FIG. 13, itis assumed to cause an eddy current to flow, as indicated by a solidline arrow in FIG. 14A. When the left-hand rule is applied to this eddycurrent, the electromagnetic force acts in a direction perpendicular tothe flow direction of the eddy current. The component of theelectromagnetic force in the direction x of the molten iron flow isgiven, as shown in a graph A in FIG. 14a. This graph exhibits occurrenceof the edge effect (the magnitude at the central portion is large, andthat at the edge portion is small).

The present inventors made extensive studies and repeated variousexperiments. The present inventors found that the edge effect could notbe fundamentally solved by an improvement of the linear motors 3A and3B, and that the structure of the injection nozzle 3 was most preferablyreplaced with a structure wherein part of the inner walls of the nozzle3 consisted of a conductive material 19 which was always in contact withthe molten iron as shown in FIG. 13.

The lines of magnetic force from the linear motors 3A and 3B aredirected from the front surface perpendicular to the drawing surfaces ofFIGS. 14a and 4b to the lower surface, and vice versa (i.e., the xdirection). When the conductive material 19 is provided to a portion(through which the lines of magnetic force flow) in a direction Yperpendicular to the injection direction Z of the molten iron and thedirection x of the lines of magnetic force, i.e., the material 19 isprovided to right and left hatched portions of the nozzle 3, as shown inFIG. 14b, eddy currents generated in these portions are also generatedinside the conductive material 19 to increase the eddy current on thesurface of the nozzle. In this case, the direction of eddy current isperpendicular to the surface of the nozzle. As indicated by a solid linearrow in FIG. 14b, the distribution is given as an elliptical shapewhose major axis is aligned in the horizontal direction. As indicated bya graph B below the ellipses, the molten iron injection (Z) component ofthe electromagnetic force takes effect, and the electromagnetic force inthe surface portion of the nozzle 3 can be increased. Therefore, theedge effect described above can be greatly improved.

The conductivity of the conductive material 19 used on the inner wallsof the nozzle 3 is preferably similar to that of the molten iron.According to experiments of the present inventors, it is recommendedthat the conductivity of the conductive material 19 is 1/10 or more thatof the molten iron.

The material for the existing injection nozzle is mainly phased silicaor alumina graphite, as described above. Alumina graphite exhibits aconductive property, but cannot have a 1/10 or more conductivity of themolten iron. Phased silica is an insulator.

ZrB₂ or carbon is recommended as a conductive material having durabilityto the molten metal. Carbon can be used with molten iron. The use of theZrB₂ which does not penetrate into the molten steel is preferable in thecase of molten steel.

A cast iron plate was inserted into the opposite inner walls in theinjection nozzle made of phased silica, and an edge effect test wasperformed. The edge effect was greatly improved, as expected, andefficiency was also improved. However, when the injection time wasprolonged, the cast iron was melted.

The thickness of the conductive material 19 is preferably large on anindustrial basis. However, the upper limit value of the thickness isdetermined by a manufacturing method. The conductive material 19 shouldbe formed at least in portions corresponding to the linear motors 3A and3B in the vertical direction, when viewed along the longitudinaldirection z of the nozzle 3. If the length of the conductive material 19exceeds the z-direction length of each of the liner motors 3A and 3B,the effect can be sufficiently enhanced. The width of each of the linearmotors 3A and 3B is preferably larger than the width of the molten ironwhen viewed in the widthwise direction Y of the nozzle 3.

According to the results which the inventors obtained by analyzingcharacteristic formulas relating to linear motors, the acting force Pwhich the linear motor applies to the molten steel and the heat quantityQ supplied from the linear motor to the molten steel, are given asequations (2) and (3) below:

    P=k.sub.1 ·f·i.sup.2 [kgf/kg]            (2)

    Q=k.sub.2 ·f.sup.2 ·i.sup.2 [°C./sec/kg](3)

where f is the input power frequency [Hz], i is the line current [A],and k₁ and k₂ are the constants.

Equations (2) and (3) are established in a low-frequency range in whichas a diamagnetic field generated by an eddy current flowing through themolten steel is smaller than a magnetic field generated by a currentflowing through an induction coil. In the high-frequency range, theforce P is not increased unlike an increase in power source capacitycaused by an increase in impedance of the linear motor. Therefore, thehigh-frequency range is not advantageous in use of the linear motor.

Equations (2) and (3) yield equations (4) and (5) below.

    F=(k.sub.1 /k.sub.2)(Q/P)                                  (4) ##EQU3## equations (4) and (5) can be rewritten as equations (6) and (7)

    f=k.sub.1 (Q/P)                                            (6) ##EQU4## where K1 and K2 are the constants.

FIG. 15 shows a detailed procedure of a method of simultaneouslycontrolling the injection rate and temperature of the molten steel byusing equations (6) and (7), and FIG. 16 is a block diagram representingthe control method.

Reference numeral 14 in FIG. 15 denotes a position detection end of thepresent invention. The position detection end 14 detects a molten steelsurface height X in the casting mold. The acting force P which thelinear motor applies to the molten steel is changed depending on adifference (X-X_(O)) between the detected molten steel surface height Xand a reference molten steel surface height (an optimal molten steelsurface height for operation) X_(O). The force P is a function of thedifference (X-X_(O)). A relation as a most suitable expression forcontinuous casting operation is defined as equation (8):

    P=ψ(X-X.sub.O)                                         (8)

Reference numeral 42 in FIG. 15 denotes an arithmetic unit whichreceives X_(O) and equation (8) in advance. The molten steel surfaceheight X detected by the position detection end 14 is transmitted to thearithmetic unit 42, and the arithmetic unit 42 calculates PIcorresponding to X.

Reference numeral 37 in FIG. 15 denotes a molten steel temperaturedetection end for detecting a molten steel temperature t. The heatquantity supplied from the linear motor to the molten steel is adjustedin accordance with a difference (t-t_(O)) between the detectedtemperature t and a reference molten steel temperature t_(O). Note thatthe heat quantity Q is defined as a function of the difference (t-t_(O))as follows:

    Q=φ(T-t.sub.O)                                         (9)

The arithmetic unit 42 of the present invention receives the referencetemperature 10 and equation (9) in advance. The actual molten steeltemperature t detected by the temperature detection end 37 istransmitted to the arithmetic unit 42, and the arithmetic unit 42calculates Q1 corresponding to t.

The arithmetic unit 42 of the present invention also receives equations(6) and (7). Therefore, the arithmetic unit 42 calculates a frequency f₁and a current i₁ which are to be input to the linear motor as follows:

    f.sub.1 =k.sub.1 (Q.sub.1 /P.sub.1) ##EQU5##

Reference numeral 24 in FIG. 15 denotes a commercial power; and 43, apower transforming unit. The arithmetic unit 42 controls the powertransforming unit 43 to cause it to transform the commercial power 24into a power having the frequency f₁ and the current i₁. The transformedpower is supplied to the linear motor, so that the force P₁ and the heatquantity Q₁ are applied to the molten steel in nozzle 3.

As described above, the force P₁ and the heat quantity Q₁ are suppliedfrom the linear motors 3A and 3B to the molten steel in accordance withsignal from the position detection end 14 and the temperature detectionend 37, so that the injection rate and temperature of the molten steelare controlled to recover the reference molten steel surface heightX_(O) and the reference molten steel temperature t_(O).

FIG. 17 is a diagram representing a seventh embodiment of injection unitaccording to the present invention. This unit has a construction similarto the unit shown in FIG. 15. However, the values P and Q are notcalculated using the aforementioned equations (8) and (9), but are inputfrom a data terminal 45.

Problems in introducing the linear motors, measures against theproblems, and the method of the embodiment have been described, thusfar. Next, the present invention will be clearly described using asimulation technique to show how effective the control with the linearmotors is compared to a conventional sliding nozzle (SN) as a moltenmetal control means. The block diagram of the control system shown inFIG. 16 is used to explain the simulation, the dynamic behavior of theactive end is approximated by dead time plus first-order lag, and thevalues in the table below are used as concrete value.

    ______________________________________                                                       SN   linear motor nozzle                                       ______________________________________                                        dead time (Td) [sec]                                                                           0.3    0.01                                                  time constant (Ts) [sec]                                                                       0.8    0.2                                                   ______________________________________                                    

Thus, there is large difference between the linear motor and the nozzlein terms of response fine.

FIG. 18 shows a simulation result of the molten metal level fluctuationstate caused by a disturbance in a casting rate 20 mpm, as a typicalexample. FIG. 19 similarly show ranges of the level fluctuation atdifferent casting rates. Thus, the range of the level fluctuation can benarrowed to less than 1/2 by use of the linear motor when comparing theSN.

Since the fluctuation range of the molten metal level is one of the mostimportant factors in the design of a thin plate continuous castingmachine, it is obvious that injection control using the linear motorbecomes more useful as the casting rate becomes higher.

Finally, FIG. 20 shows experimental data which confirms thecharacteristics of the linear motor in the case where molten steel isinjected and controlled using a linear motor having a power factorimprovement capacitor. This experiment was carried out according to thecondition of the lower-frequency power, excluding the higher-frequencypower condition, from the technical specifications of the continuouscasting machine shown in FIG. 8.

FIG. 20 is a diagram representing an experimental result of flow ratecontrol using the injection unit according to the present invention. Inthis figure, an obliquely extending curve represents the result fromcalculation, and marks X represent experimental results. Referring toFIG. 20, it is confirmed that there is a fixed relationship close to thecalculated value between the output power of the linear motor and theflow rate.

According to FIG. 20, it is confirmed that the injection rate of themolten steel is varied roughly linearly when an acting force(proportional to the square of the current) or the current value isvaried keeping the frequency constant, as characteristics of the linearmotor comprising the power factor improvement capacitor.

The nozzle used in the experiment was damaged by electromagnetic forceat more than 36 kgf of the output power of the linear motor so thatmeasurement could no longer be carried out. Thus, there is trade-offrelationship between the width of the control range and the strength ofthe nozzle. Therefore, the control range of the linear motor and thestrength of the nozzle must be carefully designed depending on thepurpose of the design of the actual equipment. In this case, use of thecontrol with the linear motor and the control of the sliding nozzle orthe stopper together is a practical and effective design.

The present invention solves practical problems when the linear motorunit is employed in the injection unit of a thin plate continuouscasting machine, by arranging a pair of linear motors to face widesurfaces of the flat nozzle and by employing a power factor improvementcapacitor.

According to the present invention, fast response injection control isrealized by introducing a linear motor which can have a small powerconsumption by elevating its efficiency. From the result of thefluctuation range was less than 1/2 that of the conventional method andthe effect becomes larger as the casting rate becomes higher.

Additionally, the efficiency of the linear motor is additionallyelevated and distribution of the electromagnetic force along the widthof the injection nozzle is uniform, so that the linear motor has an evensmaller power consumption. The yield of the products is improved, damageof the nozzle is prevented, and blocking of the nozzle is suppressed byheating the nozzle and/or molten steel with the linear motor, so that acontinuous casting is realized.

We claim:
 1. An injection apparatus in a high-speed type thin platecontinuous casting machine wherein a molten metal (2) is injected into acasting mold from a tundish (1) through a flat nozzle (3) havingY-direction long sides wider than X-direction short sides, and elongatedalong a Z-direction, characterized in that the injection apparatuscomprises:linear motors (3A, 3B), between which the long sides of saidflat nozzle (3) are interposed for generating an electromagnetic feedforce in the Z-direction along said long sides; a power source unit (24)for applying predetermined voltages or currents having a predeterminedfrequency to said linear motors (3A, 3B), to cause said linear motors(3A, 3B) to generate said electromagnetic feed force; and linear motorpower factor improving capacitors (21) connected to an electric linebetween said power source unit (24) and said linear motors (3A, 3B). 2.An injection apparatus as claimed in claim 1, comprising power controlmeans (23, 25) inserted between said power source unit (24) and saidlinear motors (3A, 3B), for controlling at least one of the voltages andcurrents supplied to said linear motors (3A, 3B) to control aZ-direction acceleration/deceleration force acting on the molten metal(2) in said flat nozzle (3).
 3. An injection apparatus as claimed inclaim 2, comprising phase switching means (22, 27) inserted between saidpower source unit (24) and said linear motors (3A, 3B) for switching thephase of the power supplied to said linear motors (3A, 3B) to switchbetween positive and negative directions of the electromagnetic feedforce of said linear motors (3A, 3B).
 4. An injection apparatus asclaimed in claim 1, wherein short side inner walls of said flat nozzle(3) essentially consist of a conductive material which is durableagainst said molten metal (2).
 5. An injection apparatus as claimed inclaim 2, wherein short side inner walls of said flat nozzle (3)essentially consist of a conductive material which is durable againstsaid molten metal (2).
 6. An injection apparatus as claimed in claim 3,wherein short side inner walls of said flat nozzle (3) essentiallyconsist of a conductive material which is durable against said moltenmetal (2).
 7. An injection apparatus as claimed in claim 4 wherein saidconductive material is ZrB₂ or carbon.
 8. An injection apparatus asclaimed in claim 5 wherein said conductive material is ZrB₂ or carbon.9. An injection apparatus as claimed in claim 6 wherein said conductivematerial is ZrB₂ or carbon.
 10. An injection apparatus as claimed inclaim 1, wherein said casting mold is a flat casting mold having atleast a pair of endless casting belts (4) wound around upstream rollers(5) and downstream rollers (5') so as to oppose each other, and lowerend portions of said linear motors (3A, 3B) extend below upper ends ofsaid upstream rollers (5).
 11. An injection apparatus as claimed inclaim 2, wherein said casting mold is a flat casting mold having atleast a pair of endless casting belts (4) wound around upstream rollers(5) and downstream rollers (5') so as to oppose each other, and lowerend portions of said linear motors (3A, 3B) extend below upper ends ofsaid upstream rollers (5).
 12. An injection apparatus as claimed inclaim 3, wherein said casting mold is a flat casting mold having atleast a pair of endless casting belts (4) wound around upstream rollers(5) and downstream rollers (5') so as to oppose each other, and lowerend portions of said linear motors (3A, 3B) extend below upper ends ofsaid upstream rollers (5).
 13. An injection apparatus as claimed inclaim 2, comprising:level detecting means (14) for detecting a moltenmetal level in a casting mold, and a control unit (30) for controllingsaid power control means (23, 25) depending on a difference between asignal from said detecting means (14) and a target molten metal level.14. An injection apparatus as claimed in claim 13, wherein the injectionapparatus comprises a stopper unit (15) provided in said tundish (1) andabove said flat nozzle (3) for controlling an injection rate of themolten metal by being moved up or down, and said control unit (30)controls said power control means (23, 25) when said difference issmaller than a predetermined value and controls said stopper unit (15)when said difference is larger than the predetermined value.
 15. Aninjection apparatus as claimed in claim 13, wherein said injectionapparatus comprises a sliding nozzle provided in the middle of said flatnozzle (3) for controlling an injection rate of the molten metal bybeing opened or closed, and said control unit (30) controls said powercontrol means (23, 25) when said difference is smaller than apredetermined value and controls said sliding nozzle when saiddifference is larger than the predetermined value.
 16. An injectionapparatus as claimed in claim 13, wherein said level detecting means(14).comprises an industrial television camera for picking up an imageof a casting mold inner wall around a target position of the moltenmetal level, and a signal processing unit for detecting a position ofthe molten metal level from the image picked up by the industrialtelevision camera and converting into a molten metal level signal. 17.An injection apparatus as claimed in claim 14, wherein said level,detecting means (14) comprises an industrial television camera (28) forpicking up an image of a casting mold inner wall around a targetposition of the molten metal level, and a signal processing unit (29)for detecting a position of the molten metal level from the image pickedup by the industrial television camera (28) and converting into a moltenmetal level signal.
 18. An injection apparatus as claimed in claim 15,wherein said level detecting means (14) comprising an industrialtelevision camera (28) for picking up an image of a casting mold innerwall around a target position of the molten metal level, and a signalprocessing unit (29) for detecting a position of the molten metal levelfrom the image picked up by the industrial television camera (28) andconverting into a molten metal level signal.
 19. An injection apparatusas claimed in claim 13, comprising:an input unit to which a heatquantity Q supplied to the molten steel by said linear motors (3A, 3B)and a force P from said linear motors (3A, 3B) acting on the moltensteel are input, a calculation unit calculating a frequency f and acurrent i using formulas

    f=K.sub.1 (Q/P)

and ##EQU6## wherein K₁ and K₂ are constants, and a power convertingunit converting a commercial power to a power having a frequency f and acurrent i according to the output of said calculation unit and supplyingthe power to the linear motors (3A, 3B).
 20. An injection apparatus asclaimed in claim 13, comprising:a temperature detecting means (37) fordetecting a temperature of the molten steel, a calculation unit forcalculating a heat quantity Q supplied to the molten steel by the linearmotors (3A, 3B) and a force P from said linear motors (3A, 3B) acting onthe molten steel from the signal of said temperature detecting means,and further calculating a frequency f and a current i using formulas

    f=K.sub.1 (Q/P)

and ##EQU7## wherein K₁ and K₂ are constants, and a power convertingunit converting a commercial power to a power having a frequency f and acurrent i according to the output of said calculation unit and supplyingthe power to the linear motors (3A, 3B).
 21. An injection apparatus asclaimed in claim 13, wherein said power source unit supplies a powerformed by superimposing a plurality of frequency bands havingfrequencies different from each other to said linear motors (3A, 3B).22. An injection apparatus as claimed in claim 13, wherein said powersource unit comprises a plurality of power supply units havingfrequencies different from each other and a switching unit for switchingthem.
 23. An injection apparatus as claimed in claim 21, wherein atleast one of said plurality of frequency bands is within a lowerfrequency range of 30 to 3000 Hz and at least another one of saidplurality of frequency bands is within a higher frequency range of 3 to450 kHz.
 24. An injection apparatus as claimed in claim 22, wherein afrequency band in at least one of said plurality of power supply unitsis within a lower frequency range of 30 to 3000 Hz and a frequency bandin at least another one of said plurality of power supply units iswithin a high frequency range of 3 to 450 kHz.
 25. An injection controlmethod for a high-speed type thin plate continuous casting machinewherein a molten metal (2) is injected into a casting mold from atundish (1) through a flat nozzle (3) having Y-direction long sideswider than X-direction short sides, and elongated along a Z-direction,comprising the steps of:providing linear motors (3A, 3B) between whichthe long sides of said flat nozzle (3) are interposed for generating anelectromagnetic feed force in a Z-direction along said long sides;providing a power source unit (24) for applying predetermined voltagesor currents having a predetermined frequency to said linear motors (3A,3B), to cause said linear motors (3A, 3B) to generate saidelectromagnetic feed force; providing linear motor power factorimproving capacitors (21) connected to an electric line between saidpower source unit (24) and said linear motors (3A, 3B); and controllingat least one of the voltages and currents supplied to said linear motors(3A, 3B) to control a Z-direction acceleration/deceleration force actingon the molten metal (2) in said flat nozzle (3).
 26. A method as claimedin claim 25, wherein the method further comprises the steps of detectinga molten metal level in the casting mold, and in said controlling step,at least one of the voltages and currents are controlled depending on adifference between the detected level and a target molten metal level.27. A method as claimed in claim 26, wherein the method furthercomprises the steps of:providing a stopper unit (15) above said flatnozzle (3) in said tundish (1) for controlling an injection rate of themolten level by being moved up or down; and interrupting saidcontrolling step and controlling said stopper unit (15) depending onsaid difference, while the difference is larger than a predeterminedlevel.
 28. A method as claimed in claim 26, wherein the method furthercomprises the steps of:providing a sliding nozzle in the middle of saidflat nozzle (3) for controlling an injection rate of the molten metal bybeing opened or closed; and interrupting said controlling step andcontrolling said sliding nozzle depending on said difference, while thedifference is larger than a predetermined level.
 29. A method as claimedin claim 25, wherein the method further comprises the steps of:detectinga molten metal level in the casting mold; detecting a temperature of themolten metal; calculating a heat quantity Q supplied to the molten steelby the linear motors (3A, 3B) and a force P from said linear motors(3A,3B) acting on the molten steel, from said detected level andtemperature; and calculating frequency f and a current i using formulas

    f=k.sub.1 (Q/P)

and ##EQU8## wherein k₁ and k₂ are constants, and in said controllingstep, a commercial power is converted to a power having a frequency fand a current i and supplied to the linear motors (3A, 3B).